Liu, 63, is a short woman with full cheeks, a rounded middle and smooth, unlined skin who bustles along the corridors of her lab like a cartoon teapot. She is, by any measure, maternal, with two grown sons. Twenty-one years ago, Liu, then an assistant professor, joined the first team in the U.S. to have produced a test-tube baby. Now she and her partners can boast an enviable success rate. In the past several years, about 40 percent of the couples that came to her group conceived, and in 2004, the center's 10,000th IVF baby was born.